Play as Mr. Tonkin:
In normal mode, move the cursor to any player in the player select screen and press L2(2), R1(3), Down, Triangle(4), Circle. A shout will confirm correct code entry.

Play as Mrs. Tonkin:
Hold L1 + R2 + Up + Square and press Circle at the player select screen. The sound of a bouncing ball will confirm correct code entry.

Play as Adversa:
Hold L1 + R2 + Up + Square and press X at the player select screen. A shout will confirm correct code entry.

Play as Mattox:
Hold L2(2), R1(3), Down, Triangle(4) and press X at the player select screen. The sound of a bouncing ball will confirm correct code entry.

Hint: Super serve:
When serving in a doubles match, walk all the way over to the opposite side and do a O,O serve. Most of the time you will ace the set.